The present invention relates to a printing mechanism in a thermal printer having a planar thermal head.
Thermal printers include a thermal head having heating elements selectively heated to melt ink on an ink donor ribbon and to apply the melted ink to a sheet of print paper for thereby forming a desired image on a print sheet. The conventional thermal printers are classified into two printer types. One of these printer types is a serial-type thermal printer having a thermal head composed of 9, 18 or 24 vertical dots, the thermal head being transversely movable in increments to print data on a sheet of print paper. The other printer type is known as a line-type thermal printer in which a line thermal head has one line of dots for printing data by feeding a sheet of print paper therepast.
The conventional serial-type thermal printer capable of color printing includes an ink donor ribbon of multiple colors movable in plural strokes in the direction in which the thermal head traverses the sheet. Each time the ink donor ribbon is moved in its stroke, the thermal head selectively heats and melts the ink donor ribbon partially to print a multiple-color image on the sheet.
To effect such a printing operation, the serial-type thermal printer has a mechanism for enabling the thermal head, the ink donor ribbon and the print sheet to move relatively to each other in plural strokes.
Such relative movement however tends to give rise to friction due to sliding engagement between the thin ink donor ribbon which has a thickness ranging from 3 to 10 microns and the thermal head. The ink donor ribbon is therefore liable to wrinkle, be broken, or elongated.
For color printing, the thermal head and the print paper must be positionally controlled with respect to each other in their relative movement in repeated strokes, since such positional control is directly related to the accuracy of combining color images printed in different colors dependent on the dot density of the thermal head or the resolution of a printed image. It has been highly difficult to obtain the desired accuracy of such positional control.
Conventional efforts to achieve the desired positional control accuracy include a mechanism for accurately feeding the thermal head and members for transmitting movement from the feeding mechanism to the thermal head, drive motor control and feedback control, precise diameters of various feed rollers employed to feed the print paper, and a correcting mechanism to guard against a skew of the ink donor ribbon.
Therefore, in order to determine the relative positions of the thermal head and the print paper, the prior thermal printers have to meet the specified tolerances of the various mechanisms, parts, and their movements, which are quite difficult to achieve. The conventional thermal printers are therefore complex in construction.
The conventional thermal printers are also disadvantageous in that they have no effective means for preventing the ink donor ribbon from wrinkling, tearing, and elongating due to the relative frictional movement of the ink donor ribbon and the thermal head.
Examples of prior art thermal printers are disclosed in Japanese Laid-Open Patent Publication No. 57-84871 published on May 27, 1982 and Japanese Laid-Open Patent Publication No. 58-20482 published on Feb. 5, 1983.